A. Autoimmune Disease
The immune system functions as the body's major defense against diseases caused by invading organisms. This complex system fights disease by killing invaders such as bacteria, viruses, parasites or cancerous cells while leaving the body's normal tissues unharmed. The immune system's ability to distinguish the body's normal tissues, or self, from foreign or cancerous tissue, or non-self, is an essential feature of normal immune system function. The loss of recognition of a particular tissue as self and the subsequent immune response directed against that tissue produces serious illness. An autoimmune disease results from the immune system attacking the body's own organs or tissues, producing a clinical condition associated with the destruction of that tissue. An autoimmune attack directed against the joint lining tissue results in rheumatoid arthritis; an attack against the conducting fibers of the nervous system results in multiple sclerosis.
Rheumatoid arthritis is one of the most common of the autoimmune diseases. Current treatments include three general classes of drugs (Schumacher, 1988): antiinflammatory agents (aspirin, non-steroidal antiinflammatory drugs and low dose corticosteroids); disease-modifying antirheumatic drugs, known as "DMARDs" (antimalarials, gold salts, penicillamine, and sulfasalazine) and immunosuppressive agents (azathioprine, chlorambucil, high dose corticosteroids, cyclophosphamide, methotrexate, nitrogen mustard, 6-mercaptopurine, vincristine, hydroxyurea, and cyclosporin A). The autoimmune diseases share a common underlying pathogenesis and the need for safe and effective therapy. None of the available drugs are completely effective, and most are limited by severe toxicity.
Recent research has established that rheumatoid arthritis and other autoimmune diseases are associated with specific cytokine abnormalities. A growing body of evidence supports the notion that IL-1 is a key mediator of inflammation in rheumatoid arthritis (RA). Elevated levels of IL-1 have been reported in synovial fluid obtained from rheumatoid arthritic joints (Smith, et al.), peripheral blood lymphocytes from RA patients have been shown to produce supranormal amounts of IL-1 in culture (Yamamura, et al.), and IL-1 has been detected at the cartilage-pannus junction (Chu, et al.), the anatomic site of articular cartilage destruction. IL-1 has been shown to increase the production of other biochemical mediators of joint inflammation, including intercellular adhesion molecules (Chin, et al.), IL-6 (Harigai, et al.; Guerne, et al.), prostaglandin E2 (O'Neill, et al.), phospholipase A (Gilman, et al.) and metalloproteases (McCachren, et al.). Macrophages isolated from the synovium of rheumatoid arthritic joints express both the IL-1 gene and the IL-1 receptor antagonist gene, indicating that an imbalance between the cytokine and its endogenous antagonist may play an etiologic role in the disease (Koch, et al.).
Investigators have shown that TNF.alpha. (tumor necrosis factor-.alpha.) also plays a significant role in the pathology of rheumatoid arthritis (Brennan). TNF.alpha. increases the severity of collagen induced arthritis in animal models (Brahn) while anti-TNF.alpha. antibody administration ameliorates collagen induced arthritis (Williams; et al.). TNF.alpha. is increased in the serum of RA patients (Holt, et al., 1992; Altomonte, et al., 1992), and both the cytokine (Chu, et al.) and its receptors have been identified in rheumatoid synovium, at the cartilage-pannus junction (Deleuran, et al.). Serum levels of both IL-1 and TNF.alpha. decline in RA patients following long term administration of the disease modifying drug sulfasalazine (Danis, et al.), suggesting that the concentrations of these cytokines reflect the clinical course of the disease.
Interleukin 6 (IL-6) is a third cytokine that appears to play a major role in the development of RA. Supranormal concentrations of this cytokine are found in both the serum (Wood, et al.; Holt, et al.) and the synovial fluid of RA patients. Synovial fluid IL-6 levels are increased during active disease (Brozik, et al.; Dasgupta, et al.; DeBennedetti, et al.), but not during remission (DeBennedetti, et al.). IL-6 concentrations vary in direct proportion to the circulating levels of IgM, IgG and IgA rheumatoid factors, but not to the total circulating levels of IgM, IgG and IgA (Sawada, et al.). IL-6 is found at the at the cartilage-pannus junction (Chu, et al.). Both synovium (Falus, et al.) and cultured synovial cells (Tan, et al.; Guerne, et al.) from RA patients produce IL-6 in vitro. The supranormal concentration of IL-6 may be an important factor in the stimulation autoantibody production (Kishimoto).
Recognition of the important role of these cytokines in autoimmune disease has fostered the development of a new generation of therapeutic agents. Proteins such as monoclonal antibodies and soluble receptors targeted against IL-1 and TNF.alpha. are currently being evaluated in clinical trials for the treatment RA and other autoimmune diseases. Preliminary results of administration of anti-IL-1 monoclonal antibodies to a small group of rheumatoid patients suggest improvement in both the clinical and laboratory manifestations of the disease (Catalano). Administration of monoclonal antibodies to TNF.alpha. has also shown encouraging early results in a group of nine patients with severe RA (Elliott).
Therapies seeking to modulate cytokine activity show promise for the treatment of autoimmune disease, but their use may prove impractical. As described above, these complex diseases are characterized by multiple cytokine abnormalities. Effective treatment could require the simultaneous administration of several agents, each targeting a specific cytokine pathway. Administration of even a single protein is likely to be limited by the potential immunogenicity of these large molecules, the need for intravenous administration and the relatively high costs of protein production.
The ideal drug for treatment of an autoimmune disease would be a plieotrophic agent that acts on each of the known aberrant cytokine pathways. In the case of rheumatoid arthritis, such a drug would inhibit IL-1, TNF.alpha. and IL-6. The development of an orally active compound that specifically modifies the production or action of these cytokines would offer a practical, cost effective contribution to the new generation of cytokine modifying drugs.
B. Organ Transplantation
Organ transplantation involving human organ donors and acceptors (allogeneic grafts) and more recently, involving non-human primate donors and human acceptors (xenogeneic grafts) has received considerable medical and scientific effort over the past two decades (Keown; Roberts; Platt). To a great extent, this effort has been aimed at eliminating, or at least reducing, the problem of transplantation rejection by the host immune system, leading to the destruction of the transplanted organ.
From follow-up studies on human transplant patients, as well as transplantation studies in animal model systems, the following features of transplantation rejection have been established. The major targets in transplant rejection are non-self allelic forms of class I and class II major histocompatibility complex (HMC) antigens (Roitt). Rejection is mediated by both antibodies and cytolytic T lymphocytes (CTLs), with the participation of CD4+ "helper" T cells (Noelle). In general, foreign class I MHC antigens stimulate CD8+ CTLs, and foreign class II MHC antigens stimulate CD4+ T cells (Roitt).
Presently, the most commonly used agents for preventing transplantation rejection include corticosteroids, cytotoxic drugs that specifically inhibit T cell activation, such as azathioprine, immunosuppressive drugs, such as cyclosporin A, and antibodies specific against donor organ foreign antigens (Briggs). All of these drug therapies are limited in effectiveness. In addition, the doses needed for effective treatment of transplantation may increase the patient's susceptibility to infection by a variety of opportunistic invaders.